Illumination system for reflective displays

ABSTRACT

An illumination system for a reflective display is particularly useful for microdisplays that use reflective displays. The light source and the reflective image display unit are mounted in a coplanar manner, thus permitting the light source and the display unit to be mounted on a single board, or even on a single substrate. The display unit may include a first light source directing light generally along a first axis and a reflective image display unit disposed with an optical axis substantially parallel to the first axis. A reflective polarizing film is disposed to direct light from the first light source to the reflective image light display unit.

FIELD OF THE INVENTION

The invention relates to reflective displays, and more particularly to acompact illumination system for a reflective display.

BACKGROUND

Many optical devices, such as microdisplays in electronic cameras andother types of display system, require illumination by a beam of lighthaving relatively uniform brightness. Generally, light sources, such asincandescent lights, arc lamps, and light emitting diodes, provide anonuniform light output that is unsuitable for direct use, so the lightis typically homogenized in a diffusing cavity before illuminating adisplay unit. The display unit is often a reflective display unit, forexample a reflective liquid crystal display panel, an array of tunablemirrors or “electronic paper”. A polarization sensitive mirror is oftenused to direct light from the light source to the display unit.

The light source and display unit are typically mounted separately fromeach other on the display system housing, and are electrically connectedvia flex circuitry. This approach results in high component andfabrication costs, and a fault in the flex circuitry or in theconnectors is often a primary failure mechanism for the display system.

Therefore, there is a need for a display system that is less expensiveto fabricate and is more reliable than current display systems.

SUMMARY OF THE INVENTION

Generally, the present invention relates to an illumination system for areflective display. The invention is believed to be particularly usefulfor microdisplays that use reflective displays. In the display system ofthe invention, the light source and display unit are mounted in acoplanar manner. This permits the light source and the display unit tobe mounted on a single board, or even on a single substrate. Thus, theassembly costs may be reduced, and the reliability increased since thesystem is simpler, has fewer components, and omits the connectors andthe flex circuit which tend to be unreliable.

One particular embodiment of the invention is an illuminated displaydevice that includes a light source directing light generally along afirst axis and a reflective image display unit disposed with an opticalaxis substantially parallel to the first axis. A reflective polarizingfilm is disposed to direct light from the first light source to thereflective image light display unit. The light source may include areflector to direct light to the reflective polarizing film.

Another particular embodiment of the invention is an illuminated displaydevice that includes light generating means for emitting diffuse,polarized light along a first direction and reflective display means formodulating reflected light with an image, the reflective display meanshaving an optical axis substantially parallel to the first axis.Reflective polarizing means are disposed to direct the diffuse,polarized light from the light generating means to the reflectivedisplay means.

In another embodiment of the invention, an optical system includes adisplay device that has a first light source directing light generallyalong a first axis and a reflective image display unit disposed with anoptical axis substantially parallel to the first axis. A reflectivepolarizing film is disposed to direct light from the first light sourceto the reflective image light display unit. A controller is coupled tothe reflective image display unit to control the image formed by thereflective image display unit. Viewing optics transport the image formedby the reflective display unit to a user.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 illustrates schematically illustrates a reflective displaysystem;

FIG. 2 illustrates a schematic of a camera having an electronicviewfinder;

FIG. 3 schematically illustrates a microdisplay connected to acontroller and a computer;

FIG. 4 schematically illustrates an optically folded reflective displaysystem;

FIG. 5 schematically illustrates an embodiment of a reflective displaysystem of the present invention;

FIGS. 6A-6D schematically illustrate embodiments of light sourcesaccording to the present invention;

FIGS. 7A and 7B schematically illustrate reflective displays withdifferent embodiments of reflector according to the present invention;

FIGS. 8A-8C schematically illustrate different embodiments of polarizingbeamsplitter according to the present invention;

FIGS. 9A-9G schematically illustrate different embodiments of reflectivedisplay according to the present invention;

FIG. 10 schematically illustrates a method of vacu-forming a doublycurved polarizing beamsplitter;

FIG. 11A schematically illustrates an embodiment of the presentinvention used in Example 1;

FIGS. 11B and 11C respectively illustrate schematic side and top viewsof the embodiment illustrated in FIG. 11A;

FIG. 12 schematically illustrates the embodiment of the invention usedin Example 2;

FIG. 13 schematically illustrates the embodiment of the invention usedin Example 3;

FIG. 14A schematically illustrates an embodiment of the presentinvention used in Example 4;

FIG. 14B illustrates a schematic side view of the embodiment illustratedin FIG. 14A;

FIG. 15A schematically illustrates an embodiment of the presentinvention used in Example 5; and

FIG. 15B illustrates a schematic side view of the embodiment illustratedin FIG. 15A.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to reflective displays and isbelieved to be particularly useful for microdisplays that employreflective display devices. Amongst the advantages provided by theinvention are a reduction in manufacturing costs for a display systemand an increased reliability.

Reflective displays are used in several types of information displaysystem. FIG. 1 illustrates basic elements of a reflective display 100. Alight source 102 transmits polarized light 104 to a reflective imagedisplay unit 106. The reflective image display unit 106 may be a liquidcrystal display (LCD) unit, for example a LCD on silicon (LCOS) display.Light 108 reflected by the reflective image display unit 106 is directedto a polarizer 110. Light 112 transmitted through the polarizer 110 isthen transmitted through viewing optics 114, which may include one ormore lenses, that transmit the image to the viewer. In this arrangement,LCD unit modulates the incident light by rotating the polarization ofsome of the incident light by 90°. Reflected light 108 whosepolarization has been rotated is transmitted by the polarizer 110 to theviewing optics 114. Reflected light whose polarization remains unrotatedis not transmitted by the polarizer 110, and is typically absorbed orreflected. The polarizer 110, therefore, separates the image light fromnon-image light. The viewing optics 114 may be, for example, aneyepiece.

The reflective image display unit 106 may also be a different type ofunit, for example an array of individually movable miniature mirrors,such as the Digital Micromirror Device™ produced by Texas Instruments,or may be based on the use of so-called “electronic paper”, such as anelectrophoretic display manufactured by E-Tek Inc., or a gyricon-baseddisplay manufactured by Xerox Corp. The invention is particularlyadvantageous for a reflective image display unit that modulates theincident light based on polarization rotation, such as an LCD, but mayalso be used for other types of reflective image display units.

Examples of where a reflective display may be used includemicrodisplays, for instance, in a viewfinder of an electronic camera.Electronic cameras include video cameras and digital cameras, and anyother device that converts an optical image to electronic form. Forexample, a video camera 200, as illustrated in FIG. 2, records an imageof an object 202. The user views an image 204 of the object 202 througha viewfinder 206 by placing his or her eye close to the viewing aperture208.

Microdisplays may also be used elsewhere, for example in head-mounteddisplays such as DVD viewers, virtual reality goggles, wearable computerdisplays and internet appliances. A general approach to using amicrodisplay is illustrated in FIG. 3, which shows microdisplay 302,which typically includes a light source, reflective image display unitand viewing optics, coupled to a controller 304. The controller 304 maybe, for example, a DVD player which is coupled to direct the image fromthe DVD player to the microdisplay 302. The controller 304 may also becoupled to, or part of, a computer system 306 to display informationfrom the computer system, for example in a heads-up display, virtualreality goggles or as a display for a wearable computer. Themicrodisplay 302 may also be used in a wearable display for a laptop orother type of computer.

It will be appreciated that reflective displays are not restricted touse in microdisplays, but may also be used in larger displays, forexample projection displays and heads-up displays.

One particular arrangement for a reflective display that may be used ina microdisplay is illustrated in FIG. 4. The reflective display 400includes a three-color light emitting diode (LED) 402 for generatinglight. Light from the LED 402 is directed to a diffuser 404 that mixesand homogenizes the color of the light that is subsequently incident onthe reflective image display unit 406. A pre-polarizer 408 polarizes thelight that has passed through the diffuser so that light of only onepolarization is incident on the reflective image display unit 406.

A brightness enhancer 410 may be placed before the diffuser 404 toenhance the brightness of the light reaching the reflective imagedisplay unit 406. For example, the brightness enhancer 410 may be a filmhaving a prismatic structure on an input surface to direct off-axislight from the LED 402 towards the axis 412, such as BEF brightnessenhancing film manufactured by 3M Company of Minnesota. Light reflectedby the BEF brightness enhancing film may be recirculated by a diffuselyreflecting cavity 413 containing the LED 402.

The brightness enhancer 410 may also be a reflective polarizing filmwhose transmission polarization state is substantially aligned with thetransmission polarization state of the pre-polarizer 408. If areflective polarizing film is used as the brightness enhancer 410, theLED 402 is advantageously enclosed within a diffusely reflecting cavity413 so that the polarization of the light reflected by the brightnessenhancer 410 may be randomized as it recirculates within the diffuselyreflecting cavity. Randomization of the polarization results in agreater fraction of the light generated by the LED 402 being transmittedby the reflective brightness enhancer 410, thus increasing the opticalefficiency of the reflective display 400. One example of a reflectivepolarizing film that may be used as brightness enhancer 410 is DBEFmultilayer optical film manufactured by 3M Company of Minnesota.

Light transmitted by the pre-polarizer 408 is polarized in the blockpolarization state of a polarizing beamsplitter 412, that is thepolarization state orthogonal to the transmission polarization state ofthe polarizing beamsplitter 412. Therefore, the light is reflected bythe polarizing beamsplitter 412 towards the reflective image displayunit 406. The reflective image display unit 406 spatially modulates theincident light 414 by polarization rotation. The reflected light 416contains light in both the block and the pass polarizations for thepolarizing beamsplitter 412. Only that light in the pass polarization ofthe polarizing beamsplitter 412, the image light 418, is transmitted tothe eyepiece 420. A clean-up polarizer 422 may be placed between thepolarizing beamsplitter 412 and the eyepiece 420 to enhance the contrastof the image viewed by the user. The use of the polarizing beamsplitter412 for reflecting the illumination light from the LED 402 and forseparating the image light 418 enables the reflective display 400 to bemore compact.

The reflective image display unit 406, the polarizing beamsplitter 412and the clean-up polarizer 422 are typically disposed within a housing424.

There are, however, certain disadvantages with the arrangement for thereflective display 400. For example, the light source, including LED402, the diffuser 404 and the pre-polarizer 408 is typically mounted ona light source board 426 while the reflective image display unit 406 ismounted on a display board 428, and the two boards 426 and 428 areseparately attached to the housing 424. Typically the two boards 426 and426 are electrically coupled using connectors and flex circuits. Thiscan add significant manufacturing and assembly cost to the displaysystem. Furthermore, the system complexity is increased and the flexcircuits lower manufacturing yields and long term reliability.

The present invention is directed to a reflective display where thelight source and the reflective image display unit are mounted in acoplanar manner. In other words, the light source is mounted so that itslight is generally directed along a first axis that is parallel to theoptical axis of the reflective image display device. An advantage ofthis approach is that the light source and the reflective image displayunit may be mounted on a shared board, thus reducing assembly costs.This permits the light source and the reflective image display unit tobe soldered to a printed circuit board using standard and relativelyinexpensive printed circuit fabrication techniques. The light source andreflective image display unit may even be formed on the same substrate,for example by evaporation or sputtering, or other fabrication method,of the appropriate materials to form an organic or inorganic LED, thusfurther reducing assembly costs. Furthermore, the flex circuits areeliminated, which not only reduces manufacturing costs, but alsoeliminates low reliability components.

A schematic view of one embodiment of the present invention isillustrated in FIG. 5. The reflective display 500 includes a lightsource 502 that generally generates light parallel to the first axis504. It will be appreciated that a light source such as an LED, tungstenbulb or the like, produces light into a large cone angle. However, thedirection of maximum intensity, also known as the chief ray, issubstantially parallel to the first axis 504.

The reflective image display unit 506 is disposed with its optical axis508 substantially parallel to the first axis, in other words is mountedcoplanar with the light source 502. Light 510 from the light source 502reflects off at least one reflecting surface, and some light reflectsoff two reflecting surfaces, before being incident on the reflectiveimage display unit 506. The reflecting surfaces may be provided by apolarizing beamsplitter 512, or a combination of a polarizingbeamsplitter 512 and another reflector 514, as is explained more fullybelow. The reflector 514 may be considered to be part of the lightsource. A clean-up polarizer 516 may be disposed to enhance thepolarization of the light transmitted through the polarizingbeamsplitter 512 to increase contrast in the image seen by the viewer.The clean-up polarizer 516 removes, through reflection or absorption,stray light of the polarization normally reflected by the polarizingbeamsplitter 512 that may have leaked through the polarizingbeamsplitter 512.

Different embodiments of a light source are illustrated in FIGS. 6A-6D.The first embodiment of light source 600, illustrated in FIG. 6A,includes a light emitter 602, which may be a three color LED array,coupled to a diffuser cavity 604. The diffuser cavity 604 may be hollowor filled with a diffusing material, and includes diffusely reflectingside walls. Light output from the cavity 604 may pass through a lens 606before reaching a diffuser 608. The combination of the diffusing cavity604 and the diffuser 608 mix and homogenize the light, thus ensuringthat the light emerging through the diffuser is uniform in color andbrightness. Light that passes through the diffuser 608 is then passedthrough a pre-polarizer 610.

The second embodiment of light source 620, illustrated in FIG. 6B,includes a light emitter 622, such as a three color LED array, coupledto a diffuser cavity 624. The diffuser cavity 624 may be hollow orfilled with a diffusing material, and includes diffusely reflecting sidewalls.

A brightness enhancer 626, for example a prismatic film or reflectivepolarizing film as described above, may be disposed to intercept lighttransmitted outwards from the cavity 624. Where the brightness enhancer626 is a prismatic film, light falling outside a specific angular rangeis reflected back to the diffuser cavity 624, while light falling withina desired angular range is transmitted. Where the brightness enhancer626 is a reflective polarizer, light in the block polarization state isreflected to the diffuser cavity 624, while light in the passpolarization state is transmitted. The light returned to the diffusingcavity is recirculated and its direction and/or polarization randomized,so that it may be transmitted through the brightness enhancer on asucceeding pass to the brightness enhancer 626.

A lens 628, such as a curved lens or a Fresnel lens, may be disposed toredirect the light transmitted by the brightness enhancer 626 so as tofall within a narrower cone angle.

The light transmitted out of the cavity 624 illuminates a diffuser 630.The combination of the diffuser 630 and the diffusing cavity 624 is usedto make the light emitted from the light source 620 uniformly bright andhave uniform color.

A third embodiment of light source 640, illustrated in FIG. 6C, employsa light emitter 642, which may be a three-color LED array, a diffuser646 and a pre-polarizer 648. Light from the light emitter 602 is coupledto the diffuser via a light guide 644. The light guide 644 may be, forexample, a solid, clear plastic pipe which traps and reflects the light,via total internal reflection at its side walls, or by reflection offside walls coated with a suitably reflective material, to an outputwindow 645. The window 645 may be capped with a diffuser or beroughened, the walls of the light guide 644 may be roughened, or thematerial within the light guide 644 may itself be diffusing, in order tohomogenize the light.

A fourth embodiment of a light source 660, illustrated in FIG. 6Demploys a light guide incorporating side extraction for directing thelight from a light emitter towards a reflective beamsplitter 512. Lightfrom a light emitter 662, for example a three-color LED unit, enters thelight guide 664. The light is extracted through the output face 666 ofthe light guide 664 by facets or other light scattering features 668 onthe disposed on the left hand face 670. The direction and divergence ofthe light 672 output through the output face 666 may be conditioned bythe extraction features 668. For example, the extraction features 668may be directed at a specific angle or partially collimated. The light672 exiting the light guide 664 may be further conditioned, for examplecollimated or partially collimated, by an array 674 of lenslets theoutput face 666.

Propagation of the light through the light guide 664 may result inmixing and homogenization of the light from the light emitter 662. Thelight 672 output from the light guide may be further homogenized by adiffuser 676. In addition, light diffusing particles may be embeddedwithin a portion, or all, of the light guide 664 to further mix andhomogenize the light.

The light 672 may be polarized by a pre-polarizer 678. The pre-polarizer678 may be a linear polarizer, for example a dichroic absorber or areflective polarizer, or may be a circular polarizer, for example acholesteric polarizer or a dichroic absorber combined with aquarter-wave retarder film.

The lenslet array 674, diffuser 676 and pre-polarizer 678 may bepositioned in a region of greatest extraction from the light guide 664in order to facilitate mixing within the guide for homogenization or toplace the outgoing light 672 at an appropriate height for illuminatingthe polarizing beamsplitter 512.

One of the advantages afforded by this embodiment 660 is its compactnessin the direction parallel to the input surface of the reflective imagedisplay unit 506.

It will be appreciated that many different types of light source may beused, in addition to variations of the four embodiments illustrated inFIGS. 6A-6D. For example, different types of light emitter may be used,such as incandescent light bulbs, halogen lamps, arc lamps, or any othersuitable light emitter. The light emitter may also include a shapedreflector, for example a parabolic reflector, in order to redirectemitted light towards the output of the light source.

The pre-polarizer may be a linear polarizer, for example a polymericmultiple layer reflective polarizing film, as described in U.S. Pat. No.5,612,820, or a wire grid polarizer, for example as described in WO94/11766, “A Reflective Polarizer”. The pre-polarizer may also be acircular polarizer, for example a cholesteric polarizer as described inU.S. Pat. No. 5,506,704. A cholesteric polarizer is particularly usefulwhere the reflective image display unit is based on the modulation ofcircularly polarized light.

Likewise, a polarizing brightness enhancer may formed from a multiplelayer reflective polarizing film, a wire grid polarizer or a cholestericpolarizer.

Any number of light emitters may be combined in a single light sourceusing this technique, for increased brightness and for covering largerarea displays. Furthermore, a reflective display may use more than onelight source.

Where light from the light source 502 is reflected off two reflectingsurfaces to reach the reflecting image display unit, a reflector 514 maybe used for the first reflection and a polarizing beamsplitter 512 usedfor the second reflection. The reflector 514 may be part of the lightsource 502.

The reflector 514 may be made of a metal-coated substrate (plastic ormetal), polished metal, a stamped metal sheet, thermoformed metal coatedfilm, a thermoformed multi-layer optical film, or other suitablereflecting material. Furthermore, the reflector 514 may be flat, asillustrated in FIG. 5, or may be curved in one or two directions toincrease illumination uniformity and efficiency. For example, areflector 714 that is curved in one direction is illustrated in FIG. 7A.The figure illustrates a light source 702 and a reflective image displaydevice 706, mounted in a coplanar fashion. The optical axes 704 and 708of the light source 702 and reflective image display device 706 are alsoillustrated. The optical axes 704 and 708 are parallel to thez-direction, and the reflective image display device 706 is translatedlaterally from the light source 702 in the x-direction. A radius ofcurvature, r, of the curved reflector 714 lies in the x-z plane.

The divergence of light in the x-z plane, emitted from the light source702, is reduced upon reflection from the curved reflector 714. The lightis directed to the polarizing beamsplitter 712 for transmission to thereflective image display device 706.

A reflector 734 that is curved in two directions is illustrated in FIG.7B. In this case, the reflector 734 has a first radius of curvaturelying in the x-z plane and a second radius of curvature lying in the y-zplane, where the y-direction is directed out of the plane of the figure,and is orthogonal to both the x and z directions. Light emitted from thelight source 702 has it divergence reduced in both the x-z and y-zplanes upon reflection from the reflector 734.

It will be appreciated that the reflector may also be singly curved witha radius of curvature in the y-z plane.

The polarizing beamsplitter 512 typically reflects light having onepolarization and transmits light having the orthogonal polarization. Thepolarization may be linear or circular. One particular example of linearpolarizer that may be used as the polarizing beamsplitter is a polymericmultiple layer polarizing film, such as DBEF manufactured by 3M Company,Minnesota. This is useful as a polarizing beamsplitter since itmaintains a high degree of extinction over a wide spectral and angularrange. Furthermore, this type of film may readily be deformed in one ortwo directions to form curved mirrors to more efficiently collect lightfrom the light source, and to lower the overall profile of theillumination system. Another type of linear polarizer also suitable foruse as the polarizing beamsplitter 512 is a wire grid polarizer.

The polarizing beamsplitter 512 may also be a circular polarizer, andmay be a cholesteric polarizer. It will be appreciated that use of acholesteric polarizer may also necessitate the introduction of a quarterwave retarder in order to convert light between linear and circularpolarization. For example, where the reflective image display unit 506operates on linearly polarized light and the light from the light sourceis also linearly polarized, then the cholesteric polarizer may beprovided with a quarter wave retarder layer on its front surface so asto circularize the polarization of the light prior to incidence on thesurface of the cholesteric polarizer. Furthermore, the quarter waveretarder linearizes the polarization of the reflected light beforepropagating to the reflective image display unit 506. Where the lightfrom the light source is circularly polarized, the reflective imagedisplay unit 506 may be provided with a quarter wave retarder at itsinput so as to linearize the polarization of the light reflected fromthe cholesteric polarizer.

The polarizing beamsplitter 512 may be flat, or curved in one or twodirections, as illustrated in FIGS. 8A-8C, which show different types ofpolarizing beamsplitter positioned close to a reflective image displayunit 806. Each polarizing beamsplitter may be provided with a clean-uppolarizer 816. The polarizing beamsplitter 812 illustrated in FIG. 8A isflat. The clean-up polarizer may be disposed immediately behind thepolarizing beamsplitter 812. The polarizing beamsplitter 822 illustratedin FIG. 8B is curved in one direction. In other words, the polarizingbeamsplitter 822 has a radius of curvature lying in the x-z plane, in amanner similar to that described above for the reflector in FIG. 7A.

Advantages of the using polarizing beamsplitter that is flat or iscurved in one dimension include the ability to directly laminate theclean-up polarizer 816 to the rear surface of the polarizingbeamsplitter 822. Furthermore, simple mechanical devices may be used toprovide the shape to the polarizing beamsplitter. For example, thepolarizing beamsplitter 822 may be formed from a sheet of material thatis constrained at its two opposite ends where the separation between theconstraints is less than the overall length of the film so that the filmbuckles to take on a curved shape. In another example, the polarizingbeamsplitter 822 may be formed from a sheet of material that is federalinto a curved slot that conforms the sheet to the desired curvature.Both of these advantages reduce manufacturing costs.

A polarizing beamsplitter 832 that is curved in two directions isillustrated in FIG. 8C. This polarizing beamsplitter 832 has a firstradius of curvature lying in the x-z plane, and a second radius ofcurvature lying in the y-z plane. A flat clean-up polarizer 816 may bepositioned above the polarizing beamsplitter 832.

The doubly curved polarizing beamsplitter 832 may be shaped byvacu-forming. Furthermore, a laminate of polarizing beamsplitter andclean-up polarizer may be vacuu-formed so that the clean-up polarizerdoes not need to be mounted separately within the display.

One particular method of vacu-forming a doubly curved polarizingbeamsplitter 832 is illustrated with respect to FIG. 10. A multilayerreflective polarizer optical film 1002 is stretched over a hole 1004 ina plate 1006. A vacuum is applied to pull the film 1002 through the hole1004. Heat is applied using a heat gun to soften the film 1002 and todeepen the sag, forming a concave surface. When cooled, the film 1002retains the concave shape. Using this technique for forming a doublycurved polarizing beamsplitter, the polarization extinction ismaintained out to the edge of the concave shape. A curved polarizingbeamsplitter 832 having an elliptical edge may also be made byvacu-forming through an elliptical hole 1004. The transmission axis ofthe curved polarizing beamsplitter may be controlled by aligning theoptical axes of the film 1002 to the major axis of the hole 1004.

A singly curved polarizing beamsplitter 822 generally shows higherpolarization extinction over a wider angular range than the doublycurved polarizing beamsplitter 832 PBS, owing to the higher range ofangles of incidence on the doubly curved surface. Thus, it becomesincreasingly more important to use a clean-up polarizer 816 with adoubly-curved polarizing beamsplitter 832. A doubly curved beamsplitterassembly may be formed by first laminating the clean-up polarizer 816 tothe polarizing beamsplitter 832 to form a lamination, and thenvacu-forming the lamination using the vacu-forming technique illustratedin FIG. 10.

It will be appreciated that the polarizing beamsplitter may be singlycurved with a radius of curvature in the y-z plane.

Different embodiments of polarizing beamsplitter are presented in thedisplay devices shown in FIGS. 9A-9G. Each display device includes alight source 902 and a reflecting image display device 906. In severalof the illustrated embodiments, the reflector 514 and the polarizingbeamsplitter 512 are formed from a single, unitary portion of reflectivepolarizer material, which reduces manufacturing costs.

In FIG. 9A, the reflector 914 and polarizing beamsplitter 912 are formedfrom a single unitary portion of the reflective polarizer material 918.The unitary portion of reflective polarizer material 918 is singlycurved, and may have different curvatures for the reflector 914 and thepolarizing beamsplitter 912. A clean-up polarizer 916 may be laminatedto the rear surface of the unitary portion of reflective polarizermaterial 918, or may be disposed elsewhere to clean up the polarizationof light transmitted through the polarizing beamsplitter 912.

In FIG. 9B, the polarizing beamsplitter 922 extends over both the lightsource 902 and the reflecting image display unit 906 to collect lightdirectly from the light source 902 and direct it to the reflective imagedisplay unit 906. The polarizing beamsplitter 922 may be doubly curved,as illustrated, or may be singly curved. A flat clean-up polarizer 926may be provided above the polarizing beamsplitter 922, or may be formedonto the rear surface of the polarizing beamsplitter.

In FIG. 9C, the reflector 934 and polarizing beamsplitter 932 are formedfrom a single unitary portion of the reflective polarizer material 938.The reflector 934 may be doubly curved, as illustrated, may be singlycurved, or may be flat.

The polarizing beamsplitter 932 may be flat, as illustrated, may besingly curved or may be doubly curved. A clean-up polarizer 936 may bedisposed to clean-up the polarization of light transmitted through thepolarizing beamsplitter 932 from the reflective image display unit 906.The clean-up polarizer 936 may be laminated or otherwise attached to thepolarizing beamsplitter 932.

Different variations of the embodiment illustrated in FIG. 9C are shownin FIGS. 9D and 9E. In FIG. 9D, the reflector 934 is doubly curved, andthe polarizing beamsplitter 932 is singly curved. In FIG. 9E, both thereflector 934 and the polarizing beamsplitter 932 are doubly curved. Itwill be appreciated that flat, singly curved and doubly curvedreflectors 934 may be combined in different ways with flat, singlycurved and doubly curved polarizing beamsplitters 932. Furthermore, theclean-up polarizer 936 may extend over both the reflector 934 and thepolarizing beamsplitter 936, for example as illustrated in FIG. 9E.

In the embodiment illustrated in FIG. 9F, the light source isdistributed. A light emitter directs light to the reflector 944. Adiffuser/polarizer 943, including a diffuser and a pre-polarizer, ispositioned between the reflector 944 and the polarizing beamsplitter 946so that light is diffused and polarized by the diffuser/polarizer 943after reflection by the reflector 944. An advantage of this embodimentis that there may be greater overlap of light beams from multipleemitters prior to diffusion and polarization, resulting in an enhancedillumination uniformity.

The invention is not limited to single light sources. Multiple lightsources may be placed in coplanar positions relative to the reflectiveimage display unit 906 to increase brightness or to improve illuminationuniformity of the reflective image display unit 906. One particularembodiment using multiple light sources is illustrated in FIG. 9G. Inthis particular embodiment, the polarization beamsplitter 952 extendsover the two light sources 902 as well as the reflective image displayunit 906. Light from the light sources 902 is reflected to thereflective image display unit 906 which modulates and reflects the lightback to the reflective polarizer 952. The modulated light is transmittedthrough the polarizing beamsplitter 952 to the viewer. A clean-uppolarizer 956 may be disposed to clean-up the light transmitted throughthe polarizing beamsplitter.

Different reflector and beamsplitter designs have been explored foreffectiveness in illuminating a reflective image display unit. The majorcharacteristics of interest in designing a reflector/beamsplittercombination include the efficiency with which light from the lightsource is directed onto the surface of the reflective image display unitwithin the display unit's acceptance cone, and the uniformity ofillumination across the reflective image display unit. An additionalparameter that was studied was the maximum height of thereflector/beamsplitter combination above the display unit. this lastparameter is important in designing display units that are used inconfined spaces, for example in a camcorder or other type of camera. Inthe examples described below, the illumination of a display unit wascalculated for a particular configuration of reflector and beamsplitter.

EXAMPLE 1

In the first example, illustrated in FIGS. 11A-11C, the light source1102 was assumed to include a light emitting diode followed by adiffuser. The light source 1102 was centered at a point approximately 5units from the center of an LCD display unit 1104. Since the displayillumination system scales linearly with size, dimensions are presentedin arbitrary “units” rather than in any particular linear measure. Thelight source 1102 was assumed to have a Lambertian, uniformly emittingsurface, having a size 1 unit×1.6 units. The short dimension was alignedparallel to direction of separation between the light source 1102 andthe LCD display unit 1104. The LCD display unit 1104 was assumed to be2.88 units×3.84 units, oriented with its long dimension parallel to theseparation direction between the LCD display unit 1104 and the LED 1102.The emitting surface of the LED 1102 was assumed to be 0.98 units higherthan the surface of the liquid crystal layer of the LCD display unit1104.

The polarizing beamsplitter 1112 was assumed to be formed as a flatsheet positioned above the LCD display unit 1104 at an angle of 40°relative to the upper surface 1104 a of the display unit 1104.

The reflector 1114 was assumed to have a “tapered box” shape, beingformed with an upper reflecting surface 1314 a, and side reflectingsurfaces 1314 b (only one side reflecting surface shown in FIGS. 11A and11B). The shape of the upper surface 1314 a was formed using an AUTOCADspline function that connected the following points in the (x,z) plane:(5.5804, −0.2035), (5.9644, 0.9476), (5.6674, 1.6398), (5.1616, 2.2553),(4.1499, 3.2774), and (3.0478, 4.5078). The tangent at the first pointwas set by the point (5.7462, 0.1190) and the tangent to the last pointwas set by the point (3.5694, 3.9642). The cross-sectional shape formedby the “tapered box” was rectangular, and the aspect ratio of therectangular cross-section was preserved throughout its length, from thelight source 1102 to the output end.

The combination of flat beamsplitter 1112 and “tapered box” reflector1114 produced the following results. The efficiency of illuminating theLCD display unit 1104 was 4.6%. The efficiency was defined as the ratioof light entering the LCD display unit 1104 within its acceptance coneangle over the total amount of light emitted by the light source 1102.The uniformity of illumination was measured by the ratio of thebrightness of the maximum of illumination intensity on the LCD displayunit 1104 over the brightness of the minimum illumination intensity onthe LCD display unit 1104. In this particular case, the max/min ratiowas 3.34. Lastly, the height, H, the maximum beamsplitter height abovethe LCD display unit 1104 required to enable this particular combinationof reflector and beamsplitter to operate most effectively, was 5.57units.

EXAMPLE 2

In the second example, illustrated in FIG. 12, the light source 1102 andLCD display unit 1104 were assumed to have the same size and relativespacing as in Example 1. The only differences between the design ofExample 1 and Example 2 were in the shapes of the reflector and thebeamsplitter. The beamsplitter 1212 was assumed to have a singly curvedshape, forming a 41° arc having a radius of curvature of 11.903 units.The reflector 1214 was assumed to be flat and oriented at 47° to theemitting surface of the light source 1102.

For this particular combination, the illumination efficiency was 3.4%,the max/min ratio was 2.8 and the height, H, was 4.53 units. The overallillumination efficiency was less than in Example 1 because the flatreflector is not as good at gathering the light from the light source1102 and presenting it to the beamsplitter for reflection to the LCDdisplay unit 1104. On the other hand, the illumination uniformity isincreased through the use of the curved beamsplitter. Also, use of thecurved beamsplitter results in a reduction in the overall height, H.

EXAMPLE 3

In the third example, illustrated in FIG. 13, the light source 1102 andLCD display unit 1104 were assumed to have the same size and relativespacing as in Example 1. The only differences between the design ofExample 1 and Example 3 were in the shapes of the reflector and thebeamsplitter. The beamsplitter 1312 was assumed to have the same shapeas in Example 2. The reflector was assumed to be a singly-curvedreflecting surface, having a curved profile matching the curved profileof the upper reflector 1114 a described for Example 1.

For this particular combination, the illumination efficiency was 4.6%,the max/min ratio was 1.93 and the height, H, was 4.53 units. The humaneye is typically able to detect a max/min ration in excess of about 2,so this design approaches the region of acceptable uniformity where theeye does not detect any nonuniformity.

EXAMPLE 4

In the fourth example, illustrated in FIGS. 14A and 14B, the lightsource 1102 and LCD display unit 1104 were assumed to have the same sizeand relative spacing as in Example 1. Furthermore, the reflector 1414was assumed to have the same “tapered box” shape as described for thereflector 1114 in Example 1. The only difference between the design ofExample 1 and Example 4 was in the shape of the beamsplitter 1412. Thebeamsplitter 1412 was assumed to have the same arcuate shape as inExample 2.

For this particular combination, the illumination efficiency was 8.2%,the max/min ratio was 1.25 and the height, H, was 4.53 units.

EXAMPLE 5

In the fifth example, illustrated in FIGS. 15A and 15B, the light source1102 and LCD display unit 1104 were assumed to have the same size andrelative spacing as in Example 1. Furthermore, the beamsplitter 1512 wasassumed to be the same as in Example 4. The reflector 1514 was assumedto have the same general “tapered box” shape as in Example 4. However,rather than having the upper surface 1514 a meet the side surfaces 1514b at right angles, the corners 1514 c between the upper surface 1514 aand sides 1514 b were curved. For this particular combination, theillumination efficiency was 6.7%, the max/min ratio was 1.52 and theheight, H, was 4.53 units. The results for the five examples aresummarized in Table I. TABLE I Summary of Illumination Characteristicsfor Different Reflector/Beamsplitter Combinations Reflector BeamsplitterMax/Min Eff. H (arb. units Ex. 1 tapered box flat 3.34 4.6% 5.57 squarecorners Ex. 2 flat single curve 2.80 3.4% 4.53 arcuate Ex. 3 singlecurve single curve 1.93 4.6% 4.53 spline arcuate Ex. 4 tapered boxsingle curve 1.25 8.2% 4.53 square corners arcuate Ex. 5 tapered boxsingle curve 1.52 6.7% 4.53 rounded arcuate corners

While various examples were provided above, the present invention is notlimited to the specifics of the illustrated embodiments. As noted above,the present invention is believed to be particularly applicable toillumination sources requiring a uniform, or substantially uniform,light output. Accordingly, the present invention should not beconsidered limited to the particular examples described above, butrather should be understood to cover all aspects of the invention asfairly set out in the attached claims. Various modifications, equivalentprocesses, as well as numerous structures to which the present inventionmay be applicable will be readily apparent to those of skill in the artto which the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

1-29. (canceled)
 30. A system as recited in claim 36, wherein thereflective polarizer is disposed between the reflective image displayunit and the viewing optics to separate light incident on the reflectiveimage display unit from light reflected by the reflective image displayunit.
 31. A device as recited in claim 36, further comprising a clean uppolarizer disposed to polarize light transmitted through the reflectivepolarizer from the reflective image display unit.
 32. A system asrecited in claim 36, further comprising a camera unit coupled to thecontroller, wherein the controller is configured to control thereflective image display unit to display an image corresponding to anobject image received by the camera.
 33. A system as recited in claim32, wherein the camera unit is a video camera.
 34. A system as recitedin claim 32, wherein the camera unit is a digital camera.
 35. A systemas recited in claim 36, further comprising a computer coupled to thecontroller to display information generated by the computer.
 36. Anoptical system, comprising: a display device including a first lightsource producing illumination light; a reflective image display unithaving an optical axis, the first light source and reflective imagedisplay unit being mounted to respectively different first and secondpositions on a substantially planar mounting surface; and a reflectivepolarizer disposed to direct light from the first light source to thereflective image light display unit; a controller coupled to thereflective image display unit to control the image formed by thereflective image display unit; and viewing optics to transport the imageformed by the reflective display unit to a user.
 37. An optical systemas recited in claim 36, wherein the first light source directs theillumination light in a direction parallel to the optical axis of thereflective image display unit.
 38. An optical system as recited in claim36, wherein the first light source comprises a prepolarizer and theillumination light produced by the light source is polarized.
 39. Anoptical system as recited in claim 36, wherein the first light sourcecomprises a diffuser, and the illumination light produces by the lightsource is diffused.
 40. An optical system as recited in claim 36,further comprising a reflector disposed to reflect light from the firstlight source to the reflective polarizer.
 41. An optical system asrecited in claim 40, wherein at least one of the reflector and thereflective polarizer is curved.
 42. An optical system as recited inclaim 36, wherein the first light source comprises a diffuser cavityhaving diffusely reflecting walls.
 43. An optical system as recited inclaim 36, wherein the first light source comprises a three color lightemitting diode array.
 44. An optical system as recited in claim 36,wherein the first light source comprises a brightness enhancer toenhance brightness of the illumination light.
 45. An optical system asrecited in claim 36, wherein the first light source comprises a lightgenerator and a light guide, light generated by the light generatorpassing through the light guide, and being emitted from the light guideas the illumination light.
 46. An optical system as recited in claim 36,wherein the first light source comprises a light generator thatgenerates the light, a diffuser to diffuse the light generated by thelight generator and a pre-polarizer to polarize light diffused by thediffuser, the light polarized by the pre-polarizer being directed to thereflective image display unit as the illumination light.
 47. Anilluminated display device, comprising: light generating means foremitting light; reflective display means for modulating reflected lightwith an image, the reflective display means having an optical axis; amount having a substantially planar mounting surface, the lightgenerating means being mounted to a first position on the mountingsurface and the reflective display means being mounted to a secondposition on the mounting surface different from the first position; andreflective polarizing means disposed to direct the light from the lightgenerating means to the reflective display means.